Silver halide photographic emulsions are usually prepared by reacting an aqueous solution of halide salt with silver salt in the presence of a protective colloid, e.g. gelatin, to produce silver halide nuclei. After physical ripening to the desired grain size and size distribution, the emulsions are subjected to chemical and spectral sensitization. Generally, in the process of manufacturing a photographic silver halide emulsion, the silver halide emulsion is usually subjected to desalting to remove water-soluble salts such as excessive silver halides, alkali nitrate and ammonium salts after completion of physical ripening. Prior to or during the chemical and spectral sensitization, the resulting water-soluble salts, e.g. sodium nitrate and excess halide during the preparation of silver halide emulsion, should be removed to prevent deleterious effects on final coating applications. It is also desirable to concentrate the washed emulsions for subsequent addition of other photographically active components.
The desalting methods include a noodle method, a dialysis method, and a flocculation precipitation method. Of these methods, the flocculation precipitation method is extensively put into practical use.
The earliest method of removing the extraneous salts is by noodle washing (U.S. Pat. Nos. 2,527,260 and 3,396,027), wherein the prepared emulsion is chilled set and broken into small fragments and subjected to a continuous water flow to remove the salt by osmosis. This technique requires a large volume of water and is very time consuming, resulting in extensive swelling of the gelatin and dilution of the remelted emulsion.
Another washing method employs the precipitation of silver halide particles by the addition of large amounts of inorganic salts, e.g. sodium or magnesium sulfates, etc. (U.S. Pat. No. 2,618,556). The interface separating the supernatant fluid and the sediment silver halide particle in such case is not well-defined, resulting in difficulty for the removal of the supernatant fluid and the loss of silver halide grains. Small molecule organic salts, e.g. sulfonated benzene, naphthalene, or their condensates with formalin, or alkyl sulfates (U.S. Pat. No. 2,527,260; GB Patent Nos. 967,624; 945,334; 1,053,670), were also employed as coagulating agents. The formation of insoluble complex between the negative charge of the coagulant and the positive amino groups of gelatin at a pH lower than the isoelectric point of gelatin, results in phase separation and coagulation of the solid silver halide particles.
Anionic polymers were also used as coagulants to generate phase separation similar to those described above by small molecule coagulants. These polymers include sulfated poly(vinyl alcohol) (U.S. Pat. No. 3,867,154); poly(vinyl sulfonate) (GB Patent No. 967,624); poly(styrene sulfonate) or its copolymers (U.S. Pat. No. 3,168,403); other sulfonated polymers (U.S. Pat. Nos. 3,241,969; 3,137,576); the copolymers of carboxylate-containing monomers, such as acrylate, methacrylates, and maleic acids (U.S. Pat. Nos. 2,565,418; 4,087,282; 4,990,439; 5,411,849; 5,486,451; Japanese 62/32445; European Patent No. 88120367.3; GB Patent No. 1,121,188). By lowering the pH of the emulsions below the isoelectric point of gelatin, complexes between the polymers and gelatin, as well as the gelatin-coated silver halide particles, are formed and separated from the clear supernatant which contains most of the soluble salts. Similar to the above anionic polymers are the modified gelatin derivatives, e.g. the covalent reaction products of gelatin with carboxylic or sulfonic acid chlorides, carboxylic anhydrides, etc. (U.S. Pat. Nos. 2,614,928; 2,614,929; 2,614,931; 3,359,110; 3,867,154; 5,411,849). The insolubility of these modified gelatin coagulants at a pH below the isoelectric point of gelatin causes precipitation of silver halide particles, and hence the soluble salt in the supernatant can be removed by decanting or centrifugation. In all the aforementioned precipitation methods, pH lowering is necessary to bring about flocculation. The extraneous ionic coagulants remain in the silver halide bottom phase, resulting in difficulty in redispersing and increase in viscosity of the subsequently redispersed emulsion and also imparting adverse effects on the photographic performance of the silver halide emulsions such as fogging.
Two other physical separation methods for the removal of soluble salts are based on membrane techniques, e.g. ultrafiltration and electrodialysis (U.S. Pat. No. 5,523,201) by use of semipermeable membranes and ion exchange membranes, respectively. Membrane fouling and the lengthy time required for desalting and difficulty in further concentration of the emulsion are possible drawbacks of these processes.
Depletion phase separation in polymer lattices containing non-adsorbing polymers have been studied extensively. Several theories have been proposed in recent years (For general references, see "Polymers at Interfaces" by G. J. Fleer, M. A. Cohen Stuart, J. M. H. M. Scheutjens, T. Cosgrove, and B. Vincent, Chapman & Hall, 1993; "Polymeric Stabilization of Colloidal Dispersion" by D. H. Napper, Academic Press, 1983) to explain such phenomena. Similar behavior is also observed with non-ionic surfactant micelles (e.g. see Progr. Colloid Polym Sci., 100, 201 (1996); Colloids and Surfaces, vol. 28, 1(1987)). The depletion phase separation is known in synthetic lattices to cause particle instability.
No working process has been described that will allow complete washing of emulsion without the need for a pH adjustment which adds to the complexity of the process and results in fogging.